Device for actuating explosives by electrical breakdown



June 1960 H. A. LIEBHAFSKY ET AL 2, 4 4

DEVICE FOR ACTUATING EXPLOSIVES BY ELECTRICAL BREAKDOWN Filed March 30, 1950 M POWDERED METAL X POWDERED EXPLOSIVE B BINDER 9 INVENTORS HERMAN A. LIEBHAFSKY, ELIZABETH L. WINSLOW, ADMINISTRATRIX FOR ARTHUR F. WINSLOW, DECEASED ATTORNEYS DEVICE FOR .ACTUATING EXPLOSIVES BY ELECTRICAL BREAKDOWN Herman A. Liebhafsky, Schenectady, N.Y., and Arthur F. Winslow, deceased, late 'of'Scotia, N.Y., thy Elizabeth L. Winslow, administra'trix, Schenectady, .N.Y., assignors to the United States of America as represented by the Secretary of the Navy Filed Mar. so, 1950, se 'No. 152,822 10 Claims. o1. Ina-2's) erally sto utilize thermally actuated squibs to electrically ignite an explosive composition. The distinguishing feature of suchtype of -sdetonator ior squib involves the conversion :of electrical energy into heat, and the use of thezheat to set off anexplosive by raising the temperature. :Normally, the ultimate function'of'the 'squib ist'o set-off an explosive charge that is much 'larger than the squib itself. In fulfilling this function"several'explosives may be involved as intermediates. As u'sedin this specification ;however, the term explosive means the first explosive ignited in the normal "operation "of the squib.

Th'e'type of squib falling within the above category may take alternate forms; for example, the conductor may bea wire "or a powdered "ingredient, e. g., carbon -or metal, or a mixture of'such'ingredient-and the explosive .placedbetweenelectrical terminals. The voltage --between the terminals :and the resistance of the conductor help to determine the characteristic'sof the squib; but the voltage is not critical, except insofar as 'it "determines the heating current. 'In-this type of squib, :the electrical resistance is perhaps the mostimpo'rtant variable. .The US. Patents Nos. 374,640 and 938,553 are representative-of-this general typeof squib.

A high tension type of explosive initiating squib \has also been suggested by the prior artwherein a spark discharge between two terminals is used -to set off the explosive. In this type of squib the electrical resistance is of little or no consequence tothe functioning-of the squib, but enormously 'high voltages are required to produce the spark discharge. Accordingly, such squibs have not had extensive application tOordnance. A squib of this general category is described in US. Patent No. 319,628. H

The detonation of an explosive by the filamentary type of squib has been controlled incertain fuzes by discharging a condenser through -a gas tube to which the'squib terminals are connected. The passage of-peak currents of sufiicient magnitude in certain types of 'fuzesis made possible only by the formation of a -hot spot on the emitting surface'of an electrode of the gas 'tube. Such electrodes, especially the filamentary types, are damaged at this-point as a consequence of the formation of the hot spot by the peak currents, and subsequent surges must strike the are at a different .place. This resultsin considerable damage to the tube each time the large peak current is drawn on the-surge. Thus, while it is i'mportantthat the tube :beproperly testedto insure that it will subsequently operate, it is very difiicult to surge test Qthe-tube without damaging it permanently.

"It .occurred that the foregoing disadvantages could be eliminated so that it 'would be possible to test thegas tubes before use without damaging them through the design of squibs that will -fire' at low voltages and -low surge currents. It was'discovere'd that a powdered metal,

2 a;pow'der'ed explosive, and a bindercan be so :mixed "together as to satisfy the above requirements. More specificall-y,:-it was discovered that a squib can be'madeconsistingofamultitudeofismall condensers, positioned between electrical terminals "that will fire by :electrical breakdown of the dielectric rat a :low consistent voltage.

Accordingly, it :is an object of this invention to pro- Wide a new and improved isquib forelectrically initiating ignition of an explosive at low current values supplied from a low voltage source.

More specifically, it is an object to provide a squib construction consisting of a multitude of small condensers located between "electrical terminals, wherein the :actuati'ng mechanism is the electrical breakdown of one or more of'these condensers ata 'low constant voltage.

It is also an object to provide :a mixture of explosive material, metal and binder so constructed and arranged 'thatdh'eametal willdistribute an-electrical field to which saidmixture 'issubje'cted so that local fields of high intensity, =i:'e., areas :having a large potential gradient are obtained.

:It is'a further object to provide a new and improved electrical squib forinitiating detonationof an explosive that will have the following advantages over the filamentary-type-of squib: ('1') will draw less current during firing, (2') 'thepeak =current will be drawn only after the squib has started to fire and thus will be incidental to the:firing mechanism, (3) will fire at lower voltages, and (4') 'will be easier to prepare.

Other objects and many of the attendant advantages .of-this invention will be'readily appreciated as the same becomes 'betterunderstood by reference to the following detailed description when considered in connection with the accompanyingdrawin'gs where:

Fig. -1 isa perspective view-of the-improved squib;

Fig. .2 is a cross section taken substantially'o'n the line 2-'-2 of Fig. 1, parts being shownin elevation, both an explosive and protcctive coating being-illustrated; and

Fig. 3-isan enlargedanddiagrammatic view illustrating the condenser principle involved herein.

'In-the-embodimentillustratedin'Figs. 1 and 2 a plug 1 of insulating material carries the lead wires '2 and 3 embedded therein and projecting therethrough in the usual manner. The conductive plates 4 and 5 aresupported-at theinner end of'the plug las by-being slightly embedded within the plug, forexample, and are connected electrically respectively to 'the lead wires 2 ands. These plates iand S'are preferably set at an acute angle, as shown, in order that the'mixo may be introduced-into the space provided-between the inner confronting surfaces "of the'plates bvmeans of a wipingmotion from the wide toward the narrow opening between the plates. Exceptionally satisfactory results have been obtained whenusing-a metal-powdenin the mix 6 nocoarser than .400 mesh by spacing the'plates 30 milsapart at the wide end with the spacing at the narrow end varying from and not thermally: 0.5 ampere at 3 volts was {passed for '5 minutes through such squib containing-a silver powder without firing it; the heat generated being 108 calories. The same squib fired when connected across 3 a 0.9 microfarad condenser charged to 25 volts; duplicate squibs not subjected to the foregoing heating current also fired at 25 volts when connected to this condenser. The energy stored in the condenser is equivalent to 6.7(10-' calories or ,4 part of the heat generated in the first experiment.

Although the electrical resistance does not determine the firing voltage of the squib mixture of this invention, an electrical conductor must be utilized in the mix to obtain the high local electrical field necessary for breakdown. The internal resistance of squibscontaining metal powder may vary from several megohms down to several ohms. The electrical resistance of the squib gives no indication of its firing voltage.

Preparation of the mix Metals such as silver, iron, nickel, titanium and zirconium may be used in the preparation of a mix that will fire without noticeable delay when used in a squib of this invention upon being subjected to a voltage of 90 volts or less. The explosive material used in the mix may comprise Tetracene which, as described in The Chemistry of Powder and Explosives by Tenney L. Davis, published in 1943, page 446, is a colorless or pale yellow, fluify material of the composition l-guanyl-4-nitrosoaminoguanyltetrazene. Other explosives which may be used are black gun powder, lead dinitro orthocresolate, lead nitrate and barium azide. The binder for the mixture of the metal and explosive may be shellac, Glyptal (trade name for synthetic resins prepared from a polyhydric alcohol and phthalic anhydride) and zapon Aquanite" grade which contains about 6.3% of nitrocellulose by weight in a suitable solvent, and sold by the Zapon Division of Atlas Powder Company, Stamford, Connecticut.

In the manufacture of a preferred mix, silver is powdered to a fineness to pass a 400 mesh sieve. (The openings in such a sieve are 0.038 mm. on the side); and anhydrous crystalline barium azide is prepared by adding an aqueous solution of barium azide containing between 0.1 and 0.2 gram of salt per cc. to about 10 times its volume of acetone. The white precipitate is quickly filtered, washed with acetone, and dried. It is important that the filtering, washing, and drying operations be carried out quickly since carbon dioxide in the air will react with the crystals to form a covering of barium carbonate which makes the crystals less reactive.

The powdered silver (2.0-3.7 grams) and the barium azide crystals (1.0-1.2 grams) are ground in an agate mortar with an agate pestle. The material adhering to the side of the mortar must be removed occasionally and mixed in with the remainder. Thorough mixing has been attained when the mix loses its mottled appearance. The grinding rate should be so regulated that uniformity is obtained in about 5 minutes. This grinding rate has been found to be important to the production of satisfactory squibs. A high yield of satisfactory squibs is obtained when the rate is regulated so that a uniform mix was obtained at the end of approximately five (5) minutes. Grinding at the rate so established is continued for approximately 15 minutes. The ground powders are now added to the binder, aquanite grade zapon (2.0-3.5 grams) which has been kept in a vacuum desiccator to remove a portion of volatile solvents. After thorough stirring the mix is set aside for approximately 10 minutes in an amyl acetate atmosphere. The stirring is continued and amyl acetate added if necessary to obtain the viscosity of a smooth paste. The composition limits for the mixture are 2.0-3.7 grams of metal, 1.0-1.2 grams of explosive and 2.0-3.5 grams of binder. This amounts to about 2 to 3 weight portions of each of the metal and binder components per .1 weight portion of explosive,

from about 40 to about 42 percent binder and about 40 r to 44 percent of metal. Since the binder contains 6.3

percent of nitrocellulose by weight, the weight of nitrocellulose in any composition can be readily determined.

The mix is introduced between the terminal plates of the squib by means of a wiping motion from the wide toward the narrow opening and thereafter baked in an oven at 110 F. for 10 minutes to remove residual solvent. After baking, the squib is capped with a suitable explosive, e.g., barium azide 10 (Fig. 3), and may be coated with binder 11 if desired.

The most sensitive silver squibs have been prepared from the following mixture: silver (powdered to pass a -400 mesh sieve), 2.5 grams; barium azide (ground fine),

1.1 grams; zapon, 2.5 grams. 7

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. An explosive initiating device comprising a pair of spaced terminals and a mixture supported between said terminals consisting essentially of from 2.03.7 grams of silver powdered to a fineness sufficient to pass a 400 mesh sieve, from 1.0-1.2 grams of powdered barium azide, and from 2.0-3.5 grams of a binder containing from 0.126 to .220 gram of nitrocellulose.

2. An explosive initiating device comprising a pair of spaced terminals and a mixture supported between said terminals consisting of 2.5 grams of silver powdered to a finess sufficient to pass a 400 mesh sieve, 1.1 grams of powdered barium azide, and 2.0-3.5 grams of a binder containing from 0.125 to .220 gram of nitrocellulose.

3. An explosive initiating device comprising at least two spaced terminals and a mixture supported between said terminals comprising: from about 40 to about 44 percent of a powderedmetal from the class consisting of silver, iron, nickel, titanium and zirconium; from about 40 to about 42 percent of a dielectric binder; and from about 14 to about 20 percent of a powdered explosive from the class consisting of black gun powder, I-guanyl- 4-nitrosoaminoguanyltetrazene, lead dinitro ortho-cresolate, lead nitrate and barium azide.

4. The device of claim 3 in which the explosive has a fineness suificient to pass a 400 mesh sieve.

5. An explosive initiating device comprising at least two spaced terminals and a mixture supported between said terminals comprising; from about 40 to about 44 percent of powdered silver; from about 14 to about 20 percent'of a powdered explosive from the class consisting of black gun powder, 1-guanyl-4-nitrosoaminoguanyltetrazene, lead dinitro ortho-cresolate, lead nitrate and barium azide; and from about 40 percent to about 42 percent of a binder containing about 6.3 percent nitrocellulose.

6. The device of claim 5 in which the silver has a fineness suflicient to pass a 400 mesh sieve.

7. An explosive initiating device comprising a pair of spaced terminals and a mixture supported between said terminals consisting essentially of about 2.0-3.7 grams of powdered silver, about 1.0-1.2 grams of powdered barium azide, and about 2.03.5 grams of a binder containing about 6.3 percent nitrocellulose.

8. The method of initiating an explosive of the class consisting of black gun powder, 1 guanyl-4-nitrosoaminoguanyltetrazene, lead dinitro ortho-cresolate, lead nitrate and barium azide which comprises mixing about 14 to 20 percent by weight of said explosive in a powdered form with about 40 to 44 percent by weight of a powdered metal from the class consisting of silver, iron, nickel, titanium and zirconium and about 40 to 42 percent by weight of a dielectric binder and subjecting said mixture to an electromotive force of not more than volts.

,9. The method of initiating an explosive of the class consisting of black gun powder, 1' guanyl-4-nitrosoaminoguanyltetrazene, lead dinitro ortho-cresolate, lead nitrate and barium azide which comprises mixing about 14 to 20 percent by weight of said explosive in a powdered form with about 40 to 44 percent by weight of powdered silver and about 40 to 42 percent by weight of a dielectric binder and subjecting said mixture to an electromotive force of not more than 90 volts.

10. The method of forming an initiating explosion for 5 detonating explosives which comprises forming a mixture of about 2.0-3.7 grams of powdered silver, 1.0-1.2 grams of powdered barium azide'and about 2.0-3.5 grams of a binder containing about 6.3 percent of nitrocellulose and subjecting said mixture to an electromotive force-of not 10 more than 90 volts.

References Cited in the file of this patent UNITED STATES PATENTS Bryant et a1. June 11, 1901 Cahuc Nov. 2, 1909 Burrows Dec. 20, 1932 FOREIGN PATENTS Great Britain of 1915 

